The present invention relates to ferroresonant transformers and, in particular, a method and apparatus for sensing the loss of regulation in a ferroresonant transformer.
A ferroresonant transformer may be considered to operate on the principle of flux gain. A primary winding provides excitation to a portion of a core which does not operate in saturation. This core section is coupled to another section of the core which links to the secondary winding. The primary and secondary windings are separated by magnetic shunts, which effectively function as a series inductance placed between the primary and secondary circuits. The core section linked to the secondary winding operates in saturation when the transformer is regulating.
The secondary circuit drives a capacitor, which, together with the inductance of the secondary circuit, forms a parallel resonant circuit, the gain of which is intended to be sufficient to drive the section of the core linked to the secondary into saturation. The secondary voltage is constant, or regulated, as long as the flux through the secondary section of the core is constant, which occurs when it is saturated.
Two factors act to reduce the flux in the secondary section, and thus pull the transformer out of regulation. First, the flux in the primary section of the core is dependent upon the magnitude of the input voltage. As the input voltage is decreased, a point will be reached at which there is insufficient gain to produce saturation flux levels in the secondary section. At this point, the transformer's output voltage begins to decrease. Additionally, as load is increased, the Q of the tuned circuit is decreased, hence the gain decreases. At some point, the gain is reduced to the point where the secondary drops out of saturation and regulation.
Because it depends on load as well as the input voltage, regulation can vary significantly depending on the load. For example, a ferroresonant transformer may be designed for operation from a nominal 120 volt source, with sufficient flux gain such that the transformer will maintain regulation at full load as the input voltage is decreased to 100 volts. However, at half load, regulation may be maintained down to an input voltage of 60 volts.
There are numerous applications for ferroresonant transformers in which it would be beneficial to be able to detect when the flux gain was becoming marginal. In other words, when the transformer was about to drop out of regulation.
For example, ferroresonant transformers are used in uninterruptible power supplies where the primary is driven by the a.c. line until a power failure or low line voltage condition occurs, at which time an inverter is used to drive the primary from a battery power source. Based on the existing art, the inverter must be started when the input voltage drops to the level at which the ferroresonant transformer would lose regulation if it were fully loaded, when in reality, it is typically not fully loaded. By detecting the actual point of loss of the necessary flux to maintain regulation, it would be possible, in many cases, to continue to operate on the a.c. line until that point was actually reached.
Sensing the loss of sufficient flux gain for regulation also would provide advance warning not available by merely monitoring the output voltage, because a ferroresonant transformer uses its stored energy to proved an output for nearly a full cycle after an input voltage failure. This advanced warning can be very valuable in some applications. For example, in computer power supplies, a few milliseconds warning of a shutdown can make the difference between an orderly shutdown and a system crash.